Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography two approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Formation of a visible image pattern by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
A survey of "direct thermal" imaging methods is given in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". Thermography is concerned with materials which are not photosensitive, but are heat sensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
According to a direct thermal embodiment operating by physical change, a recording material is used which contains a coloured support or support coated with a coloured layer which itself is overcoated with an opaque white light reflecting layer that can fuse to a clear, transparent state whereby the coloured support is no longer masked.. Physical thermographic systems operating with such kind of recording material are described on pages 136 and 137 of the above mentioned book of Kurt I. Jacobson et al.
A thermal imaging process described in European Patent Application nr. 94200612 and in European Patent Application 94201382 uses (i) a donor element comprising on a support a donor layer containing a binder and a thermotransferable reducing agent capable of reducing a silver source to metallic silver upon heating and (ii) a receiving element comprising on a support a receiving layer comprising a silver source capable of being reduced by means of heat in the presence of a reducing agent, said thermal imaging process comprising the steps of
bringing said donor layer of said donor element into face to face relationship with said receiving layer of said receiving element, PA1 image-wise heating a thus obtained assemblage by means of a thermal head or a laser, thereby causing image-wise transfer of an amount of said thermotransferable reducing agent to said receiving element in accordance with the amount of heat supplied by said thermal head and PA1 separating said donor element from said receiving element.
Prints obtained with both imaging processes mentioned above obtain densities higher than 2.5. When toning agents are added to the receiving layer of the receiving elements, neutral grey tones can be obtained. A further improvement of the neutral grey tone can be achieved by adding thermotransferable dyes to the donor layer of donor element. However, the grey tone of the image deteriorates by storage of the image at elevated temperatures (40.degree.-60.degree. C.) and/or by exposure to light.